Name: stepwise ionization of polyprotic acids
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 3 min 38 s
Channel: JoVE

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Polyprotic Acids

Monoprotische zuren bevatten, zoals fluorwaterstofzuur, bevatten een enkel ioniseerbaar proton. Daarentegen bevatten polyprotische zuren twee of meer ioniseerbare protonen. Zwavelig zuur heeft bijvoorbeeld twee ioniseerbare protonen en fosforzuur heeft er drie.Een polyprotisch zuur verliest elk van zijn protonen opeenvolgend en elke reactie heeft zijn eigen Ka.Het is gemakkelijker om een proton uit een neutraal molecuul te verwijderen dan uit een die negatief geladen is, omdat de negatieve lading de sterkte van de binding tussen het proton en het anion vergroot. Daarom is voor fosforzuur de Ka voor de verwijdering van het eerste proton, Ka1, hoger dan het tweede, Ka2, dat hoger is dan Ka3. De pH van een polyprotisch zuur kan worden geschat door alleen de eerste reactie te gebruiken als Ka1 minstens duizend keer groter is dan de daaropvolgende Ka's.De pH van een 0, 050 molaire ascorbinezuuroplossing kan bijvoorbeeld worden bepaald met behulp van de Ka1-en een ICE-tabel. Wanneer het wordt opgelost in water, valt ascorbinezuur uiteen in hydronium-en ascorbaat-ionen. De Ka1 voor deze reactie is 8 10⁻⁵ en is gelijk aan de concentratie van hydronium maal de concentratie van ascorbaatmonoanion, gedeeld door de concentratie van ascorbinezuur.Voor deze reactie kan een ICE-tabel worden opgesteld met de begin-en evenwichtsconcentraties. Vanwege de kleine waarde van x is 0, 050 min x ongeveer gelijk aan 0, 050. Door deze waarden in de uitdrukking voor Ka1 te vervangen, is de waarde voor x gelijk aan 0, 0020 molair, wat slechts 4%is van de aanvankelijke concentratie van ascorbinezuur.De benadering is dus geldig. De pH van de oplossing is gelijk aan 2, 70. De concentratie van ascorbaatdianion gevormd in de tweede stap van de reactie kan ook worden berekend met Ka2 en een ICE-tabel.Ka2 is gelijk aan 1, 6 10⁻¹² en kan worden uitgedrukt als de concentratie van hydronium maal de concentratie van ascorbaatdianion gedeeld door de concentratie van ascorbaatmonoanion. Voor deze reactie wordt de ICE-tabel ingevuld met de beginconcentratie van ascorbaatmonoanion en hydronium uit de eerste reactie, 0, 002 molair. Vanwege de kleine waarde van x kan het worden weggelaten uit de evenwichtsconcentraties van ascorbaatmonoanion en hydronium.Na het vervangen van deze waarden in de Ka2-uitdrukking en het oplossen, is x 1, 6 10⁻¹² molair. Omdat x minder is dan 5%van 0, 002 molair, is de benadering geldig. Aangezien de concentratie van hydronium-ionen die werden gegenereerd tijdens de tweede stap van ascorbinezuur-dissociatie verwaarloosbaar is, bepaalt het eerste ioniseerbare proton de pH van de oplossing.

polyprotische zuren

Zuren worden geclassificeerd op basis van het aantal protonen per molecuul dat ze bij een reactie kunnen afstaan. Zuren zoals HCl, HNO3 en HCN die één ioniseerbaar waterstofatoom in elk molecuul bevatten, worden monoprotische zuren genoemd. Hun reacties met water zijn: 


 <table style="margin-left: auto; margin-right: auto;"> 
 	<tbody> 
 		<tr> 
 			<td style="color: black; border: 1px solid black; text-align: center;">Monoprotic acids: Reactions with water</td> 
 		</tr> 

 		<tr> 
 			<td style="color: black; border: 1px solid black; text-align: center;">HCl (aq) + H2O (l) ⟶ H3O+ (aq) + Cl− (aq)</td> 
 		</tr> 

 		<tr> 
 			<td style="color: black; border: 1px solid black; text-align: center;">HNO3 (aq) + H2O (l) ⟶ H3O+ (aq) + NO3− (aq)</td> 
 		</tr> 

 		<tr> 
 			<td style="color: black; border: 1px solid black; text-align: center;">HCN (aq) + H2O (l) ⇌ H3O+ (aq) + CN− (aq)</td> 
 		</tr> 
 	</tbody> 
 </table> 

 Hoewel het vier waterstofatomen bevat, is azijnzuur, CH3CO2H, ook monoprotisch omdat alleen het waterstofatoom uit de carboxylgroep (COOH) reageert met basen: 

 <img alt="Image1" src="/files/ftp_upload/11405/11405_Figure 1.svg" style="height: 125px;"> 

 Op dezelfde manier zijn monoprotische basen basen die een enkel proton accepteren. 

 Diprotische zuren bevatten twee ioniseerbare waterstofatomen per molecuul; ionisatie van dergelijke zuren vindt plaats in twee stappen. De eerste ionisatie vindt altijd in grotere mate plaats dan de tweede ionisatie. Zwavelzuur, een sterk zuur, ioniseert bijvoorbeeld als volgt: 

 <table style="margin-left: auto; margin-right: auto;"> 
 	<tbody> 
 		<tr> 
 			<td style="color: black; border: 1px solid black; text-align: center;"></td> 
 			<td colspan="2" style="color: black; border: 1px solid black; text-align: center;">Diprotic acid: Sulfuric acid (H2SO4)</td> 
 		</tr> 
 		<tr> 
 			<td style="color: black; border: 1px solid black; text-align: center;">1st ionization</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">H2SO4 (aq) + H2O (l) ⇌ H3O+ (aq) + HSO4− (aq)</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">Ka1 = more than 102; complete dissociation</td> 
 		</tr> 

 		<tr> 
 			<td style="color: black; border: 1px solid black; text-align: center;">2nd ionization</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">HSO4− (aq) + H2O (l) ⇌ H3O+ (aq) + SO42− (aq)</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">Ka2 = 1.2 × 10−2</td> 
 		</tr> 
 	</tbody> 
 </table> 

 Dit stapsgewijze ionisatieproces vindt plaats voor alle polyprotische zuren. Koolzuur, H2CO3, is een voorbeeld van een zwak diprotisch zuur. De eerste ionisatie van koolzuur levert hydroniumionen en bicarbonaationen in kleine hoeveelheden op. 

 Eerste ionisatie: H2CO3 (aq) + H2O (l) ⇌ H3O+ (aq) + HCO3− (aq) 
 <img alt="Eq1" src="/files/ftp_upload/11405/11405_Equation_1.png" style="height: 90px;"> 

 Het bicarbonaation kan ook als een zuur werken. Het ioniseert en vormt hydroniumionen en carbonaationen in nog kleinere hoeveelheden. 

 Tweede ionisatie: HCO3− (aq) + H2O (l) ⇌ H3O+ (aq) + CO32− (aq) 

 <img alt="Eq2" src="/files/ftp_upload/11405/11405_Equation_2.png" style="height: 90px;"> 

 KH2CO3 is een factor 104 groter dan KHCO3, dus H2CO3 is de dominante producent van hydroniumionen in de oplossing. Dit betekent dat weinig van het HCO3− gevormd door de ionisatie van H2CO3 ioniseert en hydroniumionen (en carbonaationen) oplevert, en dat de concentraties van H3O+ en HCO3− praktisch gelijk zijn in een zuivere waterige oplossing van H2CO3. 

 Als de eerste ionisatieconstante van een zwak diprotisch zuur minstens een factor 20 groter is dan de tweede, is het passend om de eerste ionisatie afzonderlijk te behandelen en de daaruit voortvloeiende concentraties te berekenen voordat de concentraties van soorten die voortvloeien uit daaropvolgende ionisatie worden berekend. Deze aanpak wordt gedemonstreerd in de volgende voorbeeldoefening. 

 Ionisatie van een diprotisch zuur 

 “Koolzuurhoudend water” bevat een smakelijke hoeveelheid opgeloste kooldioxide. De oplossing is zuur omdat CO2 met water reageert en koolzuur, H2CO3, vormt. Wat zijn [H3O+], [HCO3−] en [CO32−] in een verzadigde oplossing van CO2 met een initiële [H2CO3] = 0.033? 

 <img alt="Eq3" src="/files/ftp_upload/11405/11405_Equation_3.png" style="height: 90px;"> 

 Zoals aangegeven door de ionisatieconstanten is H2CO3 een veel sterker zuur dan HCO3−, dus de stapsgewijze ionisatiereacties kunnen afzonderlijk worden behandeld. Met behulp van de verstrekte informatie wordt een ICE-tabel voor deze eerste ionisatiestap opgesteld: 

 <table style="margin-left: auto; margin-right: auto;"> 

 	<tbody> 
 		<tr> 
 			<td style="color: black; border: 1px solid black; text-align: center;"></td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">H2CO3 (aq) </td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">H3O+ (aq)</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">HCO3− (aq)</td> 
 		</tr> 
 		<tr> 
 			<td style="color: black; border: 1px solid black; text-align: center;">Initial concentration (M)</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">0.033</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">~0</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">0</td> 
 		</tr> 
 		<tr> 
 			<td style="color: black; border: 1px solid black; text-align: center;">Change (M)</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">−x</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">+x</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">+x</td> 
 		</tr> 
 		<tr> 
 			<td style="color: black; border: 1px solid black; text-align: center;">Equilibrium concentration (M)</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">0.033 − x</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">x</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">x</td> 
 		</tr> 
 	</tbody> 
 </table> 

 Het vervangen van de evenwichtsconcentraties in de evenwichtsvergelijking geeft 
 <img alt="Eq4" src="/files/ftp_upload/11405/11405_Equation_4.png" style="height: 90px;"> 

 Uitgaande van x << 0.033 en het oplossen van de vereenvoudigde vergelijking levert het resultaat op 
 
 <img alt="Eq5" src="/files/ftp_upload/11405/11405_Equation_5.png" style="height: 50px;"> 
 
 De ICE-tabel definieerde x als gelijk aan de molariteit van het bicarbonaation en de molariteit van het hydroniumion: 
 
 <img alt="Eq6" src="/files/ftp_upload/11405/11405_Equation_6.png" style="height: 90px;"> 
 
 Met behulp van de hierboven berekende bicarbonaationenconcentratie wordt de tweede ionisatie onderworpen aan een soortgelijke evenwichtsberekening: HCO3− (aq) + H2O (l) ⇌ H3O+ (aq) + CO32− (aq) 

 <img alt="Eq7" src="/files/ftp_upload/11405/11405_Equation_7.png" style="height: 180px;"> 

 Een triprotisch zuur is een zuur dat drie ioniseerbare H-atomen heeft. Fosforzuur is een voorbeeld: 

 <table style="margin-left: auto; margin-right: auto;"> 
 	<tbody> 
 		<tr> 
 			<td style="color: black; border: 1px solid black; text-align: center;"></td> 
 			<td colspan="2" style="color: black; border: 1px solid black; text-align: center;">Triprotic acid: Phosphoric acid (H3PO4)</td> 
 		</tr> 
 		<tr> 
 			<td style="color: black; border: 1px solid black; text-align: center;">1st ionization</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">H3PO4 (aq) + H2O (l) ⇌ H3O+ (aq) + H2PO4− (aq) </td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">Ka1 = 7.5 × 10−3</td> 
 		</tr> 
 		<tr> 
 			<td style="color: black; border: 1px solid black; text-align: center;">2nd ionization</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">H2PO4− (aq) + H2O (l) ⇌ H3O+ (aq) + HPO42− (aq)</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">Ka2 = 6.2 × 10−8</td> 
 		</tr> 
 		<tr> 
 			<td style="color: black; border: 1px solid black; text-align: center;">3rd ionization</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">HPO42− (aq) + H2O (l) ⇌ H3O+ (aq) + PO43− (aq)</td> 
 			<td style="color: black; border: 1px solid black; text-align: center;">Ka3 = 4.2 × 10−13</td> 
 		</tr> 
 	</tbody> 
 </table> 
 Wat de voorbeelden van diprotisch zuur betreft, is elke opeenvolgende ionisatiereactie minder uitgebreid dan de eerste, wat tot uiting komt in afnemende waarden voor de stapsgewijze zuurionisatieconstanten. Dit is een algemeen kenmerk van polyprotische zuren en opeenvolgende ionisatieconstanten verschillen vaak met een factor van ongeveer 105 tot 106. 

 Deze reeks van drie dissociatiereacties lijkt berekeningen van evenwichtsconcentraties in een oplossing van H3PO4 ingewikkeld te maken. Omdat de opeenvolgende ionisatieconstanten echter met een factor 105 tot 106 verschillen, bestaan er grote verschillen in de kleine veranderingen in concentratie die met de ionisatiereacties gepaard gaan. Dit maakt het gebruik van wiskundige vereenvoudigende aannames en processen mogelijk, zoals gedemonstreerd in de bovenstaande voorbeelden. Polyprotische basen kunnen meer dan één waterstofion accepteren. Het carbonaation is een voorbeeld van een diprotische basis, omdat het twee protonen kan accepteren, zoals hieronder weergegeven. Merk op dat, net als bij polyprotische zuren, de ionisatieconstanten afnemen met de ionisatiestap. Op dezelfde manier volgen evenwichtsberekeningen waarbij polyprotische basen betrokken zijn dezelfde benaderingen als die voor polyprotische zuren. 

 <img alt="Eq8" src="/files/ftp_upload/11405/11405_Equation_8.png" style="height: 90px;">

Polyprotische zuren

Acids are classified by the number of protons per molecule that they can give up in a reaction. Acids such as HCl, HNO3, and HCN that contain one ...

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stepwise ionization of polyprotic acids

Polyprotic acids lose hydrogen ions in steps. The number of ionizable protons in a molecule determines whether an acid is monoprotic, diprotic, or triprotic. This video also shows that polyprotic bases can accept more than one proton.

Monoprotic acids have one ionizable hydrogen atom per molecule. Examples include HCl, HNO3, and HCN. Their reactions with water form hydronium ions and the matching anion. Acetic acid, CH3CO2H, is also monoprotic because only the hydrogen in the carboxyl group reacts with bases. Monoprotic bases accept a single proton.

Diprotic acids have two ionizable hydrogen atoms, so they ionize in two steps. The first ionization is greater than the second. Sulfuric acid is a strong example, with complete first dissociation and a much smaller second dissociation. Carbonic acid, H2CO3, is a weak diprotic acid with the same stepwise pattern. Its first ionization forms hydronium and bicarbonate ions, and its second ionization converts bicarbonate into carbonate in even smaller amounts.

For carbonic acid, the first ionization constant is much larger than the second. In a pure aqueous solution of H2CO3, H3O+ and HCO3− are practically equal, while only a tiny amount of CO3 2− forms. When the first ionization constant is at least 20 times larger than the second, the two steps can be treated separately in equilibrium calculations. The example of saturated CO2 in water uses an ICE table for the first step, then a second equilibrium calculation for bicarbonate.

At equilibrium in that example, [H2CO3] is 0.033 M, [H3O+] is 1.2 × 10−4 M, [HCO3−] is 1.2 × 10−4 M, and [CO3 2−] is 4.7 × 10−11 M. This shows why the first ionization dominates the solution. It also shows how stepwise calculations can simplify polyprotic acid problems.

Triprotic acids have three ionizable hydrogen atoms. Phosphoric acid, H3PO4, is one example. Its three ionization steps produce H2PO4−, then HPO4 2−, and finally PO4 3−. Each successive ionization is less extensive than the one before it. The stepwise acid ionization constants decrease, often by a factor of about 10^5 to 10^6.

That large difference in ionization constants makes equilibrium work easier. Even though a triprotic acid has three dissociation reactions, the small concentration changes can still be handled with simplifying assumptions. Polyprotic bases follow the same idea. The carbonate ion is a diprotic base because it can accept two protons, and its ionization constants also decrease with each step.

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The equilibrium between a liquid and its vapor depends on the temperature of the system; a rise in temperature causes a corresponding rise in the vapor ...

Phase Transitions: Melting and Freezing

Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy ...

phase-transitions-sublimation-and-deposition

Phase Transitions: Sublimation and Deposition

Some solids can transition directly into the gaseous state, bypassing the liquid state, via a process known as sublimation. At room temperature and ...

Heating and Cooling Curves

When a substance&#8212;isolated from its environment&#8212;is subjected to heat changes, corresponding changes in temperature and phase of the substance ...

Phase Diagrams

A phase diagram combines plots of pressure versus temperature for the liquid-gas, solid-liquid, and solid-gas phase-transition equilibria of a substance. ...

Use models to compare the phases of matter and the physical changes they undergo.

PS.PSc.1.2

physical-and-chemical-properties-of-matter

Physical and Chemical Properties of Matter

The characteristics that enable us to distinguish one substance from another are called properties.

Physical Properties of Matter

A physical property is ...

molecular-comparison-of-gases-liquids-and-solids

Molecular Comparison of Gases, Liquids, and Solids

Particles in a solid are tightly packed together (fixed shape) and often arranged in a regular pattern; in a liquid, they are close together with no ...

Intermolecular vs Intramolecular Forces

Intermolecular forces (IMF) are electrostatic attractions arising from charge-charge interactions between molecules. The strength of the intermolecular ...

Intermolecular Forces

Atoms and molecules interact through bonds (or forces): intramolecular and intermolecular. The forces are electrostatic as they arise from interactions ...

comparing-intermolecular-forces-melting-point-boiling-point

Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

Intermolecular forces are attractive forces that exist between molecules. They dictate several bulk properties, such as melting points, boiling points, ...

Carry out investigations to compare physical and chemical properties of matter.

PS.PSc.1.3

The Atomic Theory of Matter

The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers. Leucippus and Democritus argued that all matter ...

Subatomic Particles

Dalton was only partially correct about the particles that make up matter. All matter is composed of atoms, and atoms are composed of three smaller ...

The Bohr Model

Following the work of Ernest Rutherford and his colleagues in the early twentieth century, the picture of atoms consisting of tiny dense nuclei surrounded ...

Emission Spectra

When solids, liquids, or condensed gases are heated sufficiently, they radiate some of the excess energy as light. Photons produced in this manner have a ...

The de Broglie Wavelength

In the macroscopic world, objects that are large enough to be seen by the naked eye follow the rules of classical physics. A billiard ball moving on a ...

The Uncertainty Principle

Werner Heisenberg considered the limits of how accurately one can measure properties of an electron or other microscopic particles. He determined that ...

Use models to interpret the data presented in Bohr diagrams and electron dot diagrams for neutral atoms of elements 1 through 18.

PS.PSc.1.4

Ions and Ionic Charges

In ordinary chemical reactions, the nucleus &mdash; which contains the protons and neutrons of each atom and thus identifies the element &mdash; remains ...

Use models to compare representations of atoms, ions, and isotopes.

PS.PSc.1.5

The Periodic Table

As early chemists discovered more elements, they realized that various elements could be grouped by their similar chemical behaviors. One such grouping ...

Periodic Classification of the Elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their ...

atomic-radii-and-effective-nuclear-charge

Atomic Radii and Effective Nuclear Charge

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

Ionic Radii

Ionic radius is the measure used to describe the size of an ion. A cation always has fewer electrons and the same number of protons as the parent atom; it ...

Ionization Energy

The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy ...

Electron Affinity

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



This process can be either ...

Alkali Metals

Group 1 elements are soft and shiny metallic solids. They are malleable, ductile, and good conductors of heat and electricity. The melting points of the ...

Halogens

Group 17 elements, known as halogens, are nonmetals. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine a solid. ...

Noble Gases


The elements in group 18 are noble gases (helium, neon, argon, krypton, xenon, and radon). They earned the name &ldquo;noble&rdquo; because they were ...

Use the Periodic Table as a model to predict the relative properties (metallic/nonmetallic character, ionic charge, and reactivity) and arrangement of elements based on the pattern of valence electrons in the outermost energy levels of atoms.

PS.PSc.1.6

Types of Chemical Bonds

Chemical bonding theories were pioneered by American chemist Gilbert&#xA0;N. Lewis. He developed a model called the Lewis model to explain the type and ...

Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

Bonding in Metals

Metallic bonds are formed between two metal atoms. A simplified model to describe metallic bonding has been developed by Paul Dr&#xFC;de called the ...

Construct an explanation to classify the type of chemical bond that occurs (covalent, ionic, or metallic) in a given substance.

PS.PSc.2.1

Nomenclature of Alkanes

In the late 19th-century, the number of new chemical compounds discovered increased tremendously. Hence, the necessity arose to develop a naming system ...

Nomenclature of Alkenes

The IUPAC naming system for alkenes replaces -an- with -en- in the corresponding parent alkanes. Accordingly, a simple alkene replaces the -ane suffix of ...

Nomenclature of Alkynes

Alkynes are unsaturated hydrocarbons characterized by the presence of carbon-carbon triple bonds and have a general formula CnH2n-2. The nomenclature of ...

structure-and-nomenclature-of-alcohols-and-phenols

Structure and Nomenclature of Alcohols and Phenols

Overview 
Alcohols are one of the most important functional groups in organic chemistry. The name of alcohol comes from the hydrocarbon from which it is ...

Structure and Nomenclature of Ethers

Structure and Bonding
Ethers are organic compounds with an ether functional group which is characterized by an oxygen atom connected to two &#8212; ...

IUPAC Nomenclature of Aldehydes

Aldehydes are named based on the systematic nomenclature rules set by the IUPAC. For acyclic aldehydes, the longest carbon chain containing the aldehydic ...

IUPAC Nomenclature of Ketones

Like aldehydes, ketones are named using IUPAC rules; in this case, by replacing &#8220;e&#8221; in the name of the longest hydrocarbon chain with ...

IUPAC Nomenclature of Carboxylic Acids

IUPAC names of carboxylic acids are systematically derived following a few rules discussed below.
For acyclic saturated monocarboxylic acids, the longest ...

Use models to apply International Union of Pure and Applied Chemistry (IUPAC) conventions to name and write formulas for simple compounds.

PS.PSc.2.2

Understand types, properties, and structure of matter.

Stepwise Ionization of Polyprotic Acids

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